PURPOSE: To enable arbitrary flow rate adjustment when a mixing valve is opened only on the side of water.

CONSTITUTION: This device is composed of a mixing valve 23 equipped with a mixing area for adjusting a hot water/water mixing ratio and a non-mixing area for opening only the hot water side and water side ducts, position detecting means 30 for detecting the driving position of a mixing valve element 28, flow rate control valve 36 for controlling a flow rate, mixing valve control means 35 for controlling a stepping motor 26, flow rate control valve control means 43 for controlling the flow rate control valve 36 by providing a setting inhibited zone for the valve aperture, and canceling means 42 for canceling the setting inhibited zone of the flow rate control valve control means 43 when the signal of the non-mixing area on the side of water is detected from the position detecting means 30. Thus, when only the water side of the mixing valve 23 is opened, arbitrary flow rate control is enabled by simple configuration.